Title: Engine and Combustion Modeling Developments in Fluent 6.1 Authors: F. Bedford, X. Hu, Fluent, Inc. Introduction Over the past year, developments in the commercial CFD software Fluent have expanded the capability for modeling in-cylinder flows with moving and deforming meshes (MDM), spray and spray wall interaction as well as chemical reaction. Improvements in coupling chemistry and turbulence have been implemented using a Lagrangian particle based variation of the probability density function (PDF) formulation of the conservation equations. An adaptive tabulation scheme, In-Situ Adaptive Tabulation (ISAT) [1], has been successfully implemented in Fluent 6.1 that greatly decreases the cost of integrating stiff chemical kinetic systems in multidimensional simulations. A spray wall interaction model and a crevice model have been added to Fluent 6.1, the latter specifically targeting in-cylinder simulation. Fluent 6.1 can now couple with the one dimensional gas dynamics codes that can be used, for example, to calculate intake or exhaust manifolds. Results from in-cylinder simulations with and without chemical reaction will be presented from several sources [2], [3]. Comparisons with experimental data will be shown and an outline will be given on work currently in progress that specifically targets in-cylinder computations. Non Reacting In-Cylinder Flows Fluent 6.1 has been used for numerous analyses of flow characteristics of port designs for SI engines. Two examples of such engines are presented below: the first a well instrumented optical engine from the work of Reuss et.al. from the GM Research Laboratory where detailed two dimensional Particle Image Velocimetry (PIV) measurements were made of a motored engine cycle and the second in which an existing two valve wedge style engine geometry was modified for use with direct in-cylinder fuel injection (Gasoline Direct Injection, or GDI) from the work of Suh et.al. from the University of Wisconsin-Madison. GM Research Pancake Engine Geometry The Transparent Combustion Chamber (TCC) engine is a two valve four-stroke cycle engine with at 92 mm bore and an 86 mm stroke. The top-dead-center (TDC) clearance is 12.3 mm which yields a compression ratio of 8.0:1. The engine has a single centrally located overhead camshaft with a standard symmetric profile of 248 crank angle degrees (CAD) duration and 20 CAD overlap. The intake and exhaust valves are 30 mm in diameter and open 8.9 mm at their maximum lift. Figure 1a shows the mesh from a simulation of the TCC engine with the long tubular runners which enter the cylinder nearly vertically. To facilitate swirl production, a 120 degree shroud was fitted to the valve so that the flow was blocked from exiting the short side radius of the intake port on the negative x side, as shown in Figure 1b. The PIV data is obtained by seeding the intake air with approximately 0.5 mm diameter silicone droplets while illuminating planes with a laser sheet. The laser is double pulsed with 6 ns duration pulses separated by 5 to 10 ms. The measurements shown here are ensemble means of 20 to 30 velocity field realizations. Further experimental and geometric information and can be found in Reference [2]. The Fluent 6.1 CFD simulation used approximately 100,000 cells at TDC, and 184,000 cells at Bottom Dead Center (BDC) where the intake valve closing (IVC) event occurs. The flow was started from quiescent initial conditions at 100 kPa intake manifold absolute pressure (MAP) just after the exhaust valve closing event. Thus, the exhaust port and valve were not modeled. The

discretization was second order in space and first order implicit in time and uniform time steps equivalent to 0.5 CAD were used for most of the simulation, though the time steps were reduced to 0.1 CAD for approximately 5 CAD around the intake valve opening and closing events. An initial turbulence intensity of 10 % was assumed over the entire domain. The swirl ratio (SR) was approximately 6 at IVC, much higher than typical SR values for homogeneous charge spark ignited (SI) engines. Figure 2 shows the computed results from Fluent 6.1 at BDC intake for three planes normal to the vertical (z) axis at a) 2.5 mm, b) 4.5 mm and c) 6.5mm below the top of the cylinder head. The simulation from Fluent 6.1 clearly captures the precession of the swirl center with depth and the agreement with experiment is very good. Computed and measured tumble flow shows similar agreement on a vertical plane between the two valves – the main flow rotates about the center and small rotating structures exist at the top left and bottom left corners of the cylinder. GDI Engine Geometry The GDI engine from Suh et.al. [4] has very tight geometric clearances between the piston and cylinder head which prove challenging for most moving and deforming mesh calculations. The engine has a 101.6 mm bore and an 88.4 mm stroke. The single intake valve is modeled as a standard Chevrolet small block 1.94” (49.3 mm) diameter tulip style intake. No exhaust valve or port is included in this computation. Valve lift for this simulation is 11 mm and the cam timing is assumed symmetric about 114 CAD lobe centerlines with 250 CAD duration. The bowl for this engine is hemispherical, approximately 2.8 inches in diameter and 0.7 inches in depth, cut out of the wedge shaped piston dome. The piston dome is sized so that it is approximately 1.5 mm smaller than the combustion chamber about the entire combustion chamber perimeter at TDC. The Fluent 6.1 simulation used quiescent mean flow and 10% turbulence intensity for initial conditions, second order upwinding for spatial discretization and first order implicit time differencing for temporal discretization. There were approximately 85,000 computational cells at TDC and 162,000 computational cells at BDC. The simulation required slightly less than two days to run from 360 degrees to 720 degrees on a single processor 2.2 GHz Intel Pentium, with time steps of 0.5 CAD with the layering portion of the simulation and time steps of 0.2 CAD during the portion of the simulation where automatic remeshing was occurring around the valves. The operating system was Linux and the GCC compiler was used (with full optimization) to build the version of Fluent. With the automatic remeshing capability implemented in Fluent 6.1, the tight geometric clearances between the piston and the cylinder head can be modeled with tetrahedral cells near TDC intake (near 360 CAD). As the piston enters into the fluid region defined by the combustion chamber, any skewed cells resulting from the interpenetration of the piston into the combustion chamber are automatically remeshed. The remeshing continues until the top of the piston has passed outside of the region defined by the combustion chamber (at approximately 430 CAD, or 70 degrees after TDC). After a crank angle of 430, the lower topology is no longer remeshed and is moved as a solid body – a section between the combustion chamber and the piston is then created automatically for a layering region. The automatic remeshing feature is transparent to the user and can be controlled by the parameters in the GUI. Figure 3a shows velocity vectors at a crank angle of 470, or 110 ATDC near the point where peak swirl occurs. High velocity jets from the valve curtain area are inducing considerable charge motion. Overall, the mass weighted swirl and tumble ratios show behavior more typical of SI engines, with a maximum value of swirl (-0.7) occurring at 475 CAD, or about 25 CAD after peak piston velocity (at 450 CAD), as shown in Figure 3b. Both x and y direction tumble values peak near 400 degrees with values of –0.7 and 0.8, respectively, when the valve is just opening and the velocity of the annular flow around the valve into the cylinder is a maximum. Figure 4a

shows velocity vectors on a plane through the centerline of the valve stem at TDC compression. Although there are several distinct structures underneath the valve at CA of 470, these structures dissipate over the course of the compression leaving a clockwise swirling flow in the piston bowl as shown in Figure 4b. The swirl will interact with the gasoline spray upon injection and the motion of the fuel vapor cloud will be influenced. Reacting In Cylinder Flows A single mode (mode 5) in a six-mode test cycle was simulated using Fluent 6.1. The engine is a fully instrumented Caterpillar 3400 series of 2.44 liters with a 15:1 geometric compression ratio and a bore of 13.76 cm. Further specific information regarding the engine can be found in Reference [3]. For the simulation of mode 5, the engine rpm was 1668, the start of injection was 2 CAD after TDC compression, the injected liquid used the properties of liquid benzene (c6h6) approximated as constant as a function of temperature. The Wave approach [5] was used to model secondary breakup, with model constants of B1=10 and B0 = 0.61 and the solid cone atomizer was used to model the nozzle. It should be noted that little time was spent characterizing the spray, as the main focus of the simulation was to explore the methodology of implementing ignition delay models into Fluent6.1. Because the spray was not well characterized, the overall agreement with experimental data was not as good as would be expected if the spray were represented more accurately. Figure 5a shows the spray one degree after start of injection with the color of the particles proportional to the drop diameter (initially 0.128 mm). Figure 5b shows an isosurface of fuel mass fraction colored by the temperature in the domain. Since there is little chemical reaction at this point in the simulation, most of the surface is of a color which corresponds to approximately 1000 K, or the compression temperature. The ignition delay model was of the one equation type based on the work of Hardenburg and Hase [6]. Figure 6 shows the mass averaged cylinder pressure as a function of crank angle, compared with the experimentally obtained values. Ignition delay well represented and the peak pressure is predicted accurately for both magnitude and phasing. Pressure after the burn is underpredicted in this case, however as mentioned previously this is most likely the result of the injector characterization. Summary and Future Work Fluent 6.1 represents a major step forward with respect to geometric flexibility in moving mesh calculations. Numerous validations for non-reacting flows in moving geometries clearly demonstrate the accuracy and flexibility of the approach. Additionally, computations of full chemistry with the ISAT allow a speedup of one to three orders of magnitude when compared to direct integration in a three dimensional domain for complex chemistry, and factors of two with simple chemistry. However, more validation studies for in cylinder combustion simulations are needed to evaluate the applicable range of the existing combustion models in Fluent 6.1. In cylinder specific plans for the future releases include a Lagrangian based wall film model [7], enhanced secondary breakup algorithms based on nonlinear stability theory and an implementation of the RIF [8] combustion model for in cylinder diesel engine simulation. Currently, multicomponent vaporization capability for the discrete phase model is planned for implementation in the longer term that will allow a more accurate assessment of wall film thickness and distribution for both PFI and DI engine geometries.

Acknowledgements The authors would like to gratefully acknowledge the invaluable efforts and comments of Jerry Lim, Graham Goldin, Shaoping Shi, Dan Lee and Yong Yi in the preparation of this work. References: [1] Pope, S.B. “Computationally Efficient Implementation of Combustion Chemistry using In Situ Adaptive Tabulation” , Combustion Theory and Modeling, Vol. 1, pp 41-63, 1997 [2] Reuss, et. al.., “Particle Image Velocimetry Measurements in a High Swirl Engine Used for Evaluation of Computational Fluid Dynamics Calculations”, SAE 952381, 1995 [3] Montgomery, D. and Reitz, R., “Six Mode Cycle Evaluation of the Effect of EGR and Multiple Injections of Particulate and NO Emissions from a DI Diesel Engine” , SAE 960316, 1996 [4] Suh, Edward and Rutland, C.J. , “Numerical Study of Fuel/Air Mixture Preparation in a GDI Engine” , SAE 1999-01-3657, 1999 [5] Reitz, R., “Modeling Atomization Processes in High-Pressure Vaporizing Sprays”, Atomization and Spray Technology 3 (1987) 309-337 [6] Hardenburg H. O., and Hase, F. W., “An Empirical Formula for Computing the Pressure Rise Delay of a Fuel from its Cetane Number and from the Relevant Parameters of Direct-Injection Diesel Engines” , SAE 790493, 1979 [7] Stanton, D. W. and Rutland, C.J., “Multi-dimensional modeling of thin liquid films and spray wall interactions resulting from impinging sprays” , Int. J. of Heat and Mass Transfer 41 (1998) 3037-3054 [8] Pitz, H., Barths, H. and Peters, N. “Three Dimensional Modeling of NOx and Soot Formation in DI Diesel Engines Using Detailed Chemistry Based on the Interactive Flamelet Approach ” , SAE technical paper 962057, 1996

a) b)

Figure 1:GM Research optical engine – geometry and valve configuration

a) mesh at TDC b) close up of cylinder head showing port junction

a) b) c)

Figure 2: Swirl at BDC intake for GM-Research optical engine at three locations showing the swirl center precession as a function of axial distance a) y=2.5 mm b) y=4.5 mm c) y=6.5 mm

a) b)

Figure 3: Complex piston shape with narrow clearances (Geometry courtesy of Ed Suh) a) at peak swirl and b) swirl and x, y tumble vs. crank angle

a) b)

Figure 4: Suh GDI engine at TDC compression – velocity vectors a) valve centerline b)2 mm underneath valve perpendicular to valve stem

a) b)

Figure 5: CAT DI diesel engine at 2 CAD after start of injection a) spray plume b) temperature on an isosurface of Y_fuel = 0.001

Figure 6: Mass averaged cylinder pressure for the mode 5 CAT engine simulation (data shown as

symbols – computations shown as lines)